Telescope Reviews: Coatings

PHASE COATING
Phase coating is special coating and is added to the prisms ONLY in roof prism binoculars to PREVENT destructive interference, actually referred to as constructive interference. Since roof prisms may have on surface that is below th angle of total interanl reflection they have one surface covered with a reflective mirror like material. Somewhere along the light path in the prism, the light has the potential to get out-of-phase. Phase coatings are applied to correct this problem. This is intended to act differently than anti-reflection coatings and is intended to improve the performance of the roof prisms. Standard porro prism binoculars do not use phase coating.
Links to articles on Phase Coating

See this thread discussion where Holger Mmerlitz explains phase correction and provides links for further study
Resolution and Phase Correction

All the remaining discussion here will refer to Antireflection (AR) coatings.

ANTIREFLECTION COATING

Coatings are added to porro prisms and lenses to improve the light transmission of the glass. Also they add destructive interference to cancel out the light reflected off the surfaces. This helps eliminate reflectance and ghost images, a desirable result in lenses and roof prisms, improving contrast.

An uncoated lens surface can reflect 4% of the light hitting it. Magnesium Flouride (MgF) single coating can reduce that reflected light to 1.5%. Properly applied multicoatings can reduce that reflectance to 0.5% or 0.25% per lens surface. Improperly applied multicoatings may do no better than, or not even as good as, good single coatings.

A binocular may have as many as 14 lens surfaces. With the exception of extremely high level of performace of some superior coatings, even a properly muticoated binocular can transmit at best only about 93% to 96% of incoming light. The same binocular with all 14 surfaces only single coated with MgF may transmit only 79% of the light.

Coatings are a very important product in the transmission of light. In this age of products, no one should purchase binoculars that have uncoated surfaces.

Color provides no clear indication of the quality of coatings. There are a variety of chemicals used to develop multi-coatings. Pentax SMC coatings appear purple or rose. Orion Ultraview coatings appear blue/green/purple, depending on what angle you look at them. Some Nikon coatings have a yellowish tinge to them. Single layer MgF coatings appear blue and if applied properly can be better than improperly (too thick) applied multi-coatings. Coatings that are applied too thick can appear bright green.

See this discussion for an explanation of the colors you see reflected from the coatings.
Prisms and Coatings - Differences

Generally, plain blue coatings is an indication of a single coated MgF surface. While MgF coatings are not bad, and in some instances may be better than multicoated (poorly applied) surfaces (ie. compared to the very green coatings mentioned below), single coated surfaces do reflect more light, reduce transmission and may cause internal reflections. I see more internal reflections (ghost images) in my single coated MgF binoculars than any other type of multi-coated binocular.

Coatings that have a multi-hue, blue-green, green-purple, blue-purple, purple-rose, purple-amber, can all be indications of properly applied coating. In this case, a comparison of reflections off the coatings may give some indication of reflected light and hence transmitted light. Generally, these are what I see on better binoculars.

Green coatings, especially very green coatings, may be an indication of a misapplication of the coatings during the process. Phil Harrington has been giving this advice for well over a decade. In order for coatings to work properly, they must be a precise thickness. The precision of the thickness of the coatings is measured in nanometers. Coatings that are applied too thick often end up appearing bright green in color. While the coating surface itself may appear to reflect very little light, the coatings are not performing as antireflection coatings were intended to perform. They may in fact be canceling out portions or certain wavelengths of light, allowing less light to get thru.

It is not uncommon at all to compare binoculars and find a pair with bright green coatings that simply cannot see as much light as another pair, which for all intent and purposes should produce the same result, but by virtue of better coatings can see several tenths of a magnitude deeper. Just to get a better understanding of the importance of the small fractions of magnitudes, a gain of 0.75 magnitude is approximately a 100% increase in total light.

Web Article on Coating Colors

A better indicator than color of the quality of antireflection coatings may be any reflections you can see when looking into them. It is difficult to see any reflections at all in Fujinon coatings. Look into the objectives of an Oberwerk BT100 binocular telescope and you will barely see the outline of your head. You will not see any detail in the reflected image of your face. Look into a binocular with lesser quality coatings and you may be able to see your full reflection in color with detail in your face. Any light you can see reflected coming back off the lens is not passing thru the lens. Premium AR coatings should minimize this reflected light.

You can also shine a flashlight into the objective lens of your binocular and you will see 6 or 8 reflections of the flashlight. Although this will not indicate the quality of the coatings, expect at least to see all colored reflections. White reflections may indicate uncoated glass surfaces. (Note - a cemented doublet objective may produce a white reflection as the two surfaces that are cemented together act as continuous glass and need not be coated). In this age of astronomy, no one should be observing with uncoated lenses.

Here's a simple test to compare Coatings from one model to another
Look for reflections in the coatings

Importance and value of coatings

Nikon 12x50SE coatings vs. Fujinon 10x70 FMT-SX

Coatings compared on a dozen different binoculars

Coatings differences on different models of Pentax binoculars

Q
I would make sure (binocular)are fully multicoated (oberwerk calls them broadband coatings: what ... is that!?).
A
As regards "Broadband Coatings" I asked the same question to Kevin at BigBinoculars.com recently and here was his reply :

Astromart Forums: Astro Binoculars
Re: Broadband coatings
Posted by Kevin Busarow on 4/8/2003 3:43:23 PM

Same question came up in the Binocular Astronomy Yahoo group a while back, and here's what I wrote-
======================================================
Broadband is a multilayer coating (5-7 layers) that has minimal reflectivity (.2% to .5%) across a very wide (broad) range (band) of the spectrum (450nm to 750nm). It's simply the best anti-reflection multi-coating the Chinese manufacturers can do with the equipment they have.

Common MgF2 coatings average 1.3% to 1.5% reflectivity across the same range. While the difference in efficiency with better multi-coatings is small, because binoculars have many air-to-glass surfaces, broadband on every surface (fully broadband multi-coated) can significantly improve overall light transmission. There is also a degree of improvement in contrast, as less light is scattered at each surface. You won't find this level of multicoating on the least expensive Chinese binoculars simply because of the time it takes to do the multiple layers. Some Oberwerk models have this level of multicoating now,
and all models will have it as new stock is received.
Kevin Busarow

Read the Manufacturers Explanation of levels of Coatings

see the embedded discussion at this thread
Anybody using zoom binos

While smaller exit pupils will certainly increase contrast, and as an added bonus compensate for eye aberrations, coatings are very important too. Although the 8X32 SE has a larger exit pupil than the 10X35 E2 (4mm vs. 3.5mm), the "superior" coatings help admit more light and hence help provide better contrast. Similarly, the contrast in my 15X70 '03 Oberwerk is better than my 20X80 Burgess though the exit pupil is larger (4.6mm vs. 4mm), because the Obies have better coatings.

See this discussion at the following thread
Performance of 8.5x44 vs 9x63 vs 10x50?

Q
I bought the Oberwerk 15x70 this year. Is their any obvious visual differences between the 2002 and 2003 models? I would like to know for certain if mine are the 2003 model.
A
The most obvious outwardly noticeable difference is in the coatings. The '02 model has only blue coatings on the eyepieces. The '03 model has the same coatings on the eyepieces as it does on the objectives and it gives off a green/purple color.

A
There is a significant difference in the coatings between the Oberwerk 2002 and 2003 models. While I had noted the 02 model has the most reflections off the lenses of various binoculars I was testing at the time, the new 03 model has significantly less reflections off the objective lenses. At the eye lens, the 02 models appears to be single MgF coated and the 03 model seems to have the same multicoating as the objective.

The overall view thru the new 03 model seems to be noticably brighter than the older 02 model. Viewing around the area of the Cygnus Milky Way, there was more and brighter faint star background visible in the new 03 model.

See the entire thread at
Oberwerk 15x70 2002 vs 2003

I check the coatings in my binoculars on the Moon by looking for reflections inside. If there are reflections, then the coatings arn't so great.
See this post for why you may not be able to tell very much about the coatings by Observing internal reflections of the moon

Edited by EdZ (02/16/07 07:02 AM)

Post Extras